Performances of a pneumatic tire generally deteriorate as a tread portion of the tire is worn as a result of continuous rotation under load applied thereon. Significant deterioration of drainage performance such as wet skid resistance in particular could lead to a serious situation. The wear limit of a tread portion of a tire is therefore indicated by tread wear indicators. Tread wear indicators are generally constituted of plural (six or so) bottom-up portions which are provided at a bottom of a groove in a tread portion along a circumference thereof and each formed by locally uplifting the groove bottom by a predetermined height. JIS D4230 prescribes that a tread wear indicator is to be a rise of 1.6 mm height from a groove bottom in a case of an automobile tire (excluding tires for a motor cycle, an agricultural machine, an industrial vehicle and a construction vehicle). In the case of such a tread wear indicator as described above, a user visually recognizes tread wear indicators, i.e. the rises or bottom-up portions in a groove, when the groove depth has decreased to 1.6 mm due to tire wear and the tread wear indicators are exposed at a ground-contact surface of the tread portion, thereby being warned by the tread wear indicators that it is time when the tire should be replaced.
PTL1 discloses a technique of providing a pair of bottom-up portions as tread wear indicators at an interval t of 10 to 20 mm therebetween in a groove in a tread portion of a tire such that pumping sound is generated when the pair of the bottom-up portions is exposed at a ground contact surface of the tread portion, to urge a user acoustically, as well as visually, to replace the tire.